In order to send time sensitive information, such as voice and video data, over asynchronous networks, some means must be employed or transferring timing information between the transmitting and receiving end. Timing-over-Packet (ToP) technology enables accurate timing and synchronization to be distributed across asynchronous packet infrastructures, allowing carriers to confidently support time-critical services over packet networks.
A network including ToP technology is shown in FIG. 1. The transmitter side includes a digital controlled oscillator (DCO) referenced to a local oscillator at the transmitter. The center frequency (CF) of the DCO at the transmitter side is set by a digital word. The output of the DCO depends on the set center frequency and the frequency of the local oscillator at the transmitter side. A time stamp ntx, which is the phase of master DCO clock, is transmitted to the receiver, which also contains a DCO referenced to a local oscillator at the receiver and set at the same center frequency CF as the transmitter. However, if the frequency of the local oscillator at the receiver is different from the frequency of the local oscillator at the transmitter, the output of the DCO at the receiver will be different from the output of the DCO at the transmitter. The objective is to make the output frequency of the DCO at the receiver the same as the output frequency of the DCO at the receiver. This is achieved by adjusting the center frequency of the receiver DCO to take into account the difference in local oscillator frequencies.
At receiver side, the processor compares the transmitter time stamp with a local generated time stamp from the receiver DCO. The difference in time stamp, which is called transit time, is used to adjust receiver center frequency of the receiver DCO so that it will be synchronized with the master DCO.
To minimize the impact on data transmission, the transmission rate for timing packets, i.e. packets with time stamp information, has to be limited so that it is much smaller than the network data rate. In order to provide an accurate update of the receiver DCO, a large amount of data needs to be collected, and at a low transmission rate, this can take considerable time.